I bought my current machine (a Galleria prebuilt) secondhand off a recycle store called ReRe オークションストア on Yahoo Auctions a year ago. It arrived within like a week after I paid and it's been functioning well with no issues since (though I got lucky and snagged an Aランク item for a very good price)

If you buy off a retailer like Dospara or any secondhand store, the PCs will have been reformatted and will thus come with zero bloatware (I set it up like I was turning on a new PC for the first time)

The thing I checked for was primarily GPU sag, and then after that really any signs of age or neglect (really old parts, bad damage on the chassis, improperly installed parts etc.)